Light-emitting diodes, LEDs, as a rule have relatively strong emission of light and, at the same time, high light or current efficiency and small dimensions. LEDs can emit light in a light spectrum that can be assigned, for example, to the visible range or to an infrared range or to another non-visible frequency range.
LEDs can be used in any illumination system, for example, backlighting systems of the display screens of televisions or monitoring systems. It is possible through the use of LEDs to make available a lighting system with light distribution that is more uniform than, for example, a lighting system with neon light.
The brightness of an LED can be controlled, for example, by controlling the value of a current through the LED. However, this can lead to a change of the spectral color of the LED. Another possibility for controlling LEDs is the use of pulse width-modulated, PWM, or pulse density-modulated, PDM, control signals through which the LEDs are alternately switched on and off. In this case, the brightness of the LEDs is dependent on the timewise average value of the current through the LEDs, which is usually kept essentially constant in this case.
FIG. 8 shows an example of a traditional control circuit for controlling an LED string 50, which comprises three LEDs 51, 52 and 53, for example. A switch 26 and a current source 25 that are connected in series with the LED string 50 are provided for control. A supply terminal VS and a reference potential terminal VB are provided to supply voltage to the circuit. The switch setting of switch 26 is controlled via a control signal CTL.
In FIG. 9, a hypothetical control signal CTL is represented in a signal-time diagram. Here, the control signal CTL is a pulse width-modulated signal with duty cycle TON/TCYC.
The flow of current for the LED string 50 is switched completely on or completely off by the switching of switch 26 in dependence on the pulse width-modulated control signal CTL. This leads to high current peaks during switching, which has an unfavorable effect on the electromagnetic compatibility, EMC, or the electromagnetic effect, EME.
Moreover, nearly the entire supply voltage is present between the supply terminal VS and the reference potential terminal VB via the switch 26 in its open, nonconducting state. For this reason, it is necessary to dimension switch 26 or the control circuit appropriately, so that it can withstand the potential applied voltage.